1. Field of the Invention
This invention relates to a novel liquid crystal substance and more particularly it relates to an azoxy liquid crystal substance having an optically active group, and a chiral smectic liquid crystal composition containing the same.
2. Description of the Prior Art
A number of azoxy group- or azo group-containing liquid crystal substances have already been known, and it has also been well known that among these substances, those which exhibit nematic phase within a preferable temperature range can be used for TN (Twisted Nematic) type display mode.
On the other hand, as to optically active group-containing azoxy compounds and azo compounds, compounds expressed by the following formula (A) or (B) have been known: ##STR3## wherein R represents methyl, ethyl, propyl or butyl group (see Japanese patent publication Nos. Sho 53-36452/1978 and Sho 53-35937/1978); and ##STR4## wherein R represents methyl, ethyl, propyl or butyl group (see Japanese patent publication Nos. Sho 53-36451/1978 and Sho 53-36453/1978). Among these compounds, however, the only one which has been shown to exhibit a liquid crystal phase is a compound of the formula (A) wherein R represents ethyl group (clearing point: 68.degree. C.), and yet the liquid crystal phase which it exhibits is presumed to be cholesteric phase.
These nematic and cholesteric liquid crystal substances have been utilized for TN type display mode. As to liquid crystal display elements, TN type display mode has now been most widely employed, but TN type display elements are inferior in response speed to emissive display elements (such as electroluminescence, plasma display, etc.); hence various improvements therein have been attempted, but it does not appear that a large extent of improvement has been achieved. Thus, various liquid crystal display devices based on a different principle, in place of TN type display elements, have been attempted, and among them there is a display mode utilizing a ferroelectric liquid crystal (N. A. Clerk et al, Applied Phys. lett., 36, 899 (1980)). This mode utilizes the chiral smectic C phase (hereinafter abbreviated to SC* phase) or the chiral smectic H phase (hereinafter abbreviated to SH* phase), and it is preferred that such phases be present in the vicinity of room temperature.
The present inventors have searched for various azoxy compounds having an optically active group, mainly in order to develop liquid crystal substances suitable to such a mode, i.e. a light-switching mode utilizing ferroelectric liquid crystals, and as a result have found that novel optically active substances different from the above compounds of the formula (A) and the formula (B) exhibit a chiral smectic phase.